When a controversial American academic, Paul Yelland, visits Oxford's Department of Criminology to lecture on his genetics-based "Theory of Dangerousness," angry protesters amass, chanting "No platform for racists!" But whether Yelland's theory is crackpot racism or groundbreaking science doesn't matter for the shaken, slightly inebriated American. Escorted back to his room by Professor Frazer and student Nina Clemens, he is angry and paranoid. Later that night, he is found dead, strangled with a noose made from his own tie.
With the local headlines proclaiming, "Lynched!" and racial tensions running high in Oxford, Chief Superintendent Innocent urges Lewis and Hathaway to tread carefully but quickly. Could the killer be one of the local anti-racist extremists? Might Anne Rand, the ambitious, publicity-hungry head of the department, have engineered it to bring media attention to her department? Or might Nina Clemens, a black student who was deeply disturbed by Yelland's unsavory ideology, have exacted revenge? Even the department's unassuming administrator Lilian Hunter seems desperate to hide a secret.
Old-fashioned police methodology rather than genetics reveals the leads that Lewis and Hathaway need in the form of fingerprints on Yelland's doorknob and on the opened envelope of a missing letter. Finding the letter should lead the way to the killer. But when one of the criminology department's own turns up dead, Lewis and Hathaway must trust their own instincts, and each other, to assess true dangerousness and catch a murderer.
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Warning: Contains significant plot spoilers
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Visiting professor Paul Yelland gives an incendiary talk at Oxford University on "criminal dangerousness." A crowd of protestors takes exception to his view that likely criminals can be identified before they have committed a crime. The demonstrators are disruptive but nonviolent, and Yelland survives hostile questions and later too many glasses of wine at a reception.
But he doesn't survive an evening alone in his guest apartment. For that night he is found hanging by his necktie, no longer a danger to anyone.
On questioning the faculty members who hosted Yelland's talk, Lewis and Hathaway discover that the professor was set up: Department of Criminology head Anne Rand invited Yelland to speak in order to spark controversy that would raise the profile of her department. Meanwhile, her husband, Robert Fraser, used his contacts with an anti-racist group to guarantee a vociferous greeting for Yelland.
Two other members of the department were also involved. Professor Andrew Lipton knew Yelland from their graduate school days, which helped lure the controversial academic to Oxford. And department administrator Lilian Hunter — wittingly or not — served Yelland the drugged wine that made him unusually groggy.
As if suspicion of murder were not enough, Anne receives an anonymous letter that accuses her husband of adultery with one of his students, Nina Clemens, a promising scholar of West Indian heritage. Anne confronts Robert and later Nina, forcing them to break off the affair.
The plot thickens when Lewis and Hathaway discover that Lilian is Yelland's illegitimate daughter and that she is the one who prompted Anne to invite Yelland to speak. Lilian wanted to confront him for abandoning her as a child along with her mother, who recently killed herself. The night of the murder, she slipped a tell-all letter from her mother under Yelland's door, but it disappeared from the scene.
Then things take a tragic turn when Nina is found bludgeoned to death. Her housemate Emily Robinson confesses that she sent anonymous racist text messages to Nina and that she also mailed the revelatory letter to Anne about Nina's affair. Her motive was rivalry for the attention of a young man, Will Pascoe, who was infatuated with Nina. But Emily claims she stopped short of murder.
Now that everyone has a motive for at least one of the homicides, the case suddenly turns on fiber evidence on Nina's body. The blow that killed her came from a clothbound book, and her corpse was wrapped in a rug before being disposed.
Hathaway gets this news as he is waiting to speak with Yelland's old colleague from graduate school, Andrew. The detective immediately notices a discolored patch on Andrew's living room floor that matches the size of the rug found near Nina's body.
Despite being a professor of criminology, Andrew doesn't stall for long before confessing. Years earlier, he admits, he and Yelland vied for the love of the same woman, who would become Lilian's mother when Yelland got her pregnant. Following her suicide, Andrew vowed to get even and gave Lilian, whom he had taken on as a protégée, the idea to approach Anne about inviting Yelland to speak. After breaking into Yelland's apartment and strangling him, Andrew found the letter that Lilian had left and removed it.
Later, Nina was visiting Andrew for a tutorial and the professor was suddenly seized with fear that she had found the letter and had taken it. He struggled with her, she fell and hit her head, and he finished the job with a sturdy tome.
He might have read some of his own books on criminal behavior to rein in his homicidal impulses — or at least avoid getting caught.